In paper making machines, in particular tissue paper, a Yankee cylinder is normally used to dry the paper before winding it in a reel. One or more doctor blades co-act with the Yankee cylinder, with one of their edges pressed against the cylindrical surface of the Yankee cylinder. The purpose of these doctor blades is to detach the ply of cellulosic fibers from the Yankee cylinder and to carry out any other operations, for example cleaning. The Yankee cylinder can be provided with a single doctor blade or several doctor blades placed in sequence.
Doctor blades are also used in combination with other types of cylinders or rollers, for example drying rollers.
The doctor blade of a creping device is usually inserted inside a doctor blade holder that forms a housing seat for the doctor blade. The doctor blade is pressed against the cylindrical surface of the rotating cylinder with a system that can be rigid or flexible. In flexible systems the fingers that support the doctor blade and that define the housing seat for the doctor blade are thrust by a pressurized fluid chamber against the cylindrical surface of the rotating cylinder. The flexibility of the pressurized fluid chamber makes the doctor blade flexible. Flexible systems are described, for example, in the U.S. Pat. Nos. 3,859,690 and 3,955,531. These systems have some advantages, in particular as regards the possibility of adapting the profile of the doctor blade to the shape of the cylindrical surface. However, they have some limits. In particular, they are prone to breakages and can be difficult to maintain and use.
The fingers are generally fixed integral to one another by means of a “plate” that extends in transverse direction to the axis of the machine. This “plate” is generally relatively thin, so that it is sufficiently elastic to allow each finger to take a different angular position with respect to the position of adjacent fingers. The assembly of the fingers connected to one another (defining a pressure plate) is mounted on the load-bearing structure, typically a beam, by means of a hole made in each finger, into which a rod of a length corresponding to the doctor blade holder is inserted. When a finger breaks or is damaged and must be replaced, the rod is removed and the pressure plate is then detached. The damaged fingers must be separated from the pressure plate, replaced and reconnected with the plate. To avoid the lengthy machine downtime necessary for these mounting and removal operations, spare pressure plates are normally provided, so that when a finger of the operating doctor blade holder breaks, the whole assembly of fingers is replaced with a single operation and the damaged pressure plate is repaired separately.
In other systems, known as rigid, the housing seat for the doctor blade is fixed rigidly to a support or beam, which is thrust toward the cylinder by suitable actuators. These systems are of much simpler construction than flexible systems, but also have some drawbacks, in particular due to the difficulty of adapting the shape of the doctor blade to the cylindrical surface of the cylinder. This latter can in fact have a shape that differs from the perfectly cylindrical geometrical shape and can have a camber, with a larger diameter in the intermediate portion and a smaller diameter at the ends of the cylinder. This makes it necessary for the doctor blade to be thrust against the cylindrical surface with very high pressures, resulting in increased frictions and torques, as well as wear.
Normally the support, or beam, of the doctor blade holder is mounted on the stationary load-bearing structure so as to rotate around an axis substantially parallel to the rotation axis of the cylinder with which the doctor blade mounted in the doctor blade holder co-acts. The doctor blade approaches the cylindrical surface of the cylinder or roller by means of a pivoting movement of the beam around the respective rotation axis. For this purpose, the beam is supported by means of bearings that define restraint, substantially representing hinges. The beam thus mounted is prone to deformations and vibrations in a non-negligible manner under the thrust generated by the contact pressure between the doctor blade and the cylinder. This deformation can cause difficulties in the operation of the cylinder and of the doctor blade co-acting therewith.
There is therefore a need to produce doctor blade holders that completely or partly overcome the drawbacks and the limits of doctor blade holders of the current art.